Single-gender groups are advocated for women because substance abuse treatment programs tend to be male dominated both in numbers and in style. Although there is a paucity of empirical work that directly addresses this assumption, a body of literature that indirectly supports it is reviewed. Male and female substance abusers report different histories and courses for their disorders and display different needs and characteristics in treatment settings. Men and women may benefit from different treatment approaches with less structure required for women, particularly problem drinkers. Moreover, specific characteristics such as victimization, sexual orientation, sex-role conflict and degree of antisocial traits may be important mediating factors in whether single-gender groups are optimal. Social psychological research illustrates the effect of gender composition of groups for men and women. In general, mixed groups are associated with more variation in interpersonal style for men but are restrictive for women who show more variation in style in single-gender groups. These findings are suggestive of testable hypotheses concerning the effects of single and mixed gender programming. Such investigations have the potential of enhancing treatment efficacy.